1. Field of the Invention
The present invention relates to a self-luminous device (or an EL display device) manufactured by forming a light emitting element (such as an EL (Electro Luminescence) element) on a substrate, and an electric machine having the self-luminous device as a display (display unit). The light emitting element here is also called an OLED (Organic Light Emitting Device).
The light emitting element has a layer containing an EL material that can provide EL (Electro Luminescence: the luminescence generated by applying an electric field) (hereinafter referred to as EL layer), in addition to an anode and a cathode. The luminescence generated from an EL material includes light emission (fluorescence) upon returning from the singlet excitation to the ground state and light emission (phosphorescence) upon returning from the triplet excitation to the ground state. The self-luminous device of the present invention can use both types of light emitting elements with one type containing fluorescent EL materials and the other type containing phosphorescent EL materials.
2. Description of the Related Art
The technology for forming a TFT on a substrate has made a great progress in recent years, and application of the thus formed TFT to an active matrix display device is being developed. In particular, a TFT formed of a polysilicon film has a field mobility (often abbreviated as mobility) higher than that of a conventional TFT that is formed of an amorphous silicon film, and hence is capable of operating at high speed.
An active matrix self-luminous device has a pixel structure generally as the one shown in FIG. 3. In FIG. 3, reference symbol 301 denotes a TFT functioning as a switching element (hereinafter referred to as switching TFT), 302, a TFT functioning as an element for controlling current supplied to an EL element 303 (current controlling element) (hereinafter referred to as current controlling TFT), and 304, a capacitor (storage capacitor). The switching TFT 301 is connected to a gate wiring 305 and a source wiring (data line) 306. The current controlling TFT 302 has a drain region connected to the EL element 303 and has a source region connected to a power supply line 307.
When the gate wiring 305 is selected, a gate of the switching TFT 301 is opened, a data signal from the source wiring 306 is stored in the capacitor 304, and a gate of the current controlling TFT 302 is opened. After the gate of the switching TFT 301 is closed, the gate of the current controlling TFT 302 is kept open due to the electric charges stored in the capacitor 304 and the EL element 303 emits light during the gate is opened. How much light is emitted from the EL element varies depending on the amount of current flowing therethrough.
In other words, an analog-driven gray scale display, the amount of light emitted from the EL element varies as a result of control over the amount of current flowing into the gate of the current controlling TFT 302 by means of a data signal inputted from the source wiring 306.
FIG. 4A is a graph showing a transistor characteristic of the current controlling TFT. Denoted by reference symbol 401 is a curve showing a so-called Id-Vg characteristic (also called Id-Vg curve), where Id represents drain current and Vg represents gate voltage. With this graph, one can tell how much current will flow at a given gate voltage.
When driving the EL element, the voltage within an area indicated by a dotted line 402 around the curve of the Id-Vg characteristic is usually used. The area enclosed by the line 402 is enlarged in FIG. 4B.
In FIG. 4B, the shaded area is called a sub-threshold region. The term actually denotes a region in which the gate voltage is about the same as a threshold voltage (VTH). When the gate voltage changes in this region, the drain current is changed exponentially. The current control is made by using the gate voltage of this region.
A data signal inputted in a pixel when the switching TFT 301 of FIG. 3 is opened is first stored in the capacitor 304, and the signal serves as the gate voltage for the current controlling TFT 302 without undergoing any change. At this point, the gate voltage determines the drain current in a 1:1 ratio in accordance with the Id-Vg characteristic shown in FIG. 4A. Thus a given amount of current flows in the EL element 303 in accordance with the data signal, and the EL element emits light in an amount corresponding to this given amount of current.
As described above, the amount of light emitted from the EL element is controlled by means of the inputted signal, and the control over the amount of light to be emitted provides gray scale display. This is a method so-called analog gray scale in which gray scale display is provided by variations in signal amplitude.
However, the analog gray scale method has a drawback and it is helpless against fluctuation in characteristic of TFTs. As an example, let's assume the case where the Id-Vg characteristic of one switching TFT differs from the Id-Vg characteristic of its adjacent pixel's switching TFT allocated for the same scale as the one switching TFT in gray scale display (which means shift toward plus or minus on the whole).
The switching TFTs in this case differ from each other in drain current, depending on how much the characteristics differ between the TFTs. This makes the gate voltage applied to one current controlling TFT in one pixel differ from the gate voltage applied to the other current controlling TFT in the adjacent pixel. Therefore different amounts of current flow in the two EL elements thereof to cause them to emit different amounts of light, with the result that the EL elements intended for the same scale in gray scale display now cannot play their intended roles.
Even when the same gate voltage is applied to the current controlling TFTs in the adjoining pixels, the current controlling TFTs cannot output the same amount of drain current if they are different from each other in Id-Vg characteristic. Moreover, as is apparent from FIG. 4A, the gate voltage used here is in the region where a change in gate voltage exponentially changes the drain current. Therefore if there is even a slightest difference in Id-Vg characteristic, equality in gate voltage does not always assure equality in amount of current outputted. Then it can be expected that EL elements in adjoining pixels may emit light in amounts greatly different from each other.
Since the fluctuation between switching TFTs and the fluctuation between current controlling TFTs affect synergistically, acceptable fluctuation in Id-Vg characteristic is in an even narrower range in actuality. The analog gray scale method is thus extremely sensitive to the fluctuation in characteristic of the TFTs, which forms an obstacle toward achieving multi-color display in conventional active matrix self-luminous devices.